Industrial coatings are surface protective coatings (paint coatings) applied to substrates and typically cured or crosslinked to form continuous films for decorative purposes as well as to protect the substrate. A protective coating ordinarily comprises an organic polymeric binder, pigments, and various paint additives, where the polymeric binder acts as a fluid vehicle for the pigments and imparts rheological properties to the fluid paint coating. Upon curing or crosslinking, the polymeric binder hardens and functions as a binder for the pigments and provides adhesion of the dried paint film to the substrate. The pigments may be organic or inorganic and functionally contribute to opacity and color in addition to durability and hardness. Protective coatings which contain little or no opacifying pigments are described as clear coatings. The manufacture of protective coatings involves the preparation of a polymeric binder, mixing of component materials, grinding of pigments in the polymeric binder, and thinning to commercial standards.
Epoxy resins are particularly desirable for use in protective surface coating materials as a vehicle or polymeric binder for the pigments, fillers, and other additives where the epoxy resins advantageously provide toughness, flexibility, adhesion, and chemical resistance. Water-dispersed coating compositions containing epoxy resins are highly desirable for can coating compositions and particularly useful for interior surfaces of containers. Coatings for the interior of soft drink and beer cans, for instance, are critical due to taste sensitivity wherein such can coatings must not alter the product taste of beverages in the containers. Taste problems can occur in a variety of ways such as by leaching of coating components into the beverage, or by absorption of flavor by the coating, or sometimes by chemical reaction, or by perhaps some combination thereof.
Container coating technology frequently utilizes an epoxy resin which has been grafted with acrylic monomers, styrene, and methacrylic acid. This grafted epoxy resin is prepared in solvent, usually butyl cellosolve, and n-butanol, to maintain low processing viscosities and then reduced with water by a direct or inverse let down procedure. Although cured film properties are highly desirable, such coatings suffer from the fact that sizeable amounts of solvents are required to obtain good performance. High molecular weight epoxy resins typically require 25% to 50% solvent (based on total solids plus organic solvent) before reducing with amine and water.
Epoxy based can coatings comprising a carbon grafted acrylic chain are disclosed is commonly assigned U.S. Pat. No. 4,212,781 which teaches a carbon grafting process involving solvent polymerization at moderate temperatures with high levels of peroxide initiator to produce a carbon-graft polymer. The high solvent levels, however, invariably carry over to the aqueous dispersion when the resulting polymers are dispersed into water to produce a VOC (volatile organic compounds) level considerably above 2 and typically between 3 and 4 pounds volatile organic compounds per gallon of resin solids. The acrylic grafted epoxy is particularly useful when utilized with a coreactive crosslinking melamine crosslinker.
Aqueous coating compositions based on microgel resin reaction product obtained by the esterification reaction of epoxy resin with carboxyl group containing vinyl polymer are disclosed in U.S. Pat. No. 4,897,434 where major amounts of high molecular weight epoxy are esterified in organic solvent with the carboxyl vinyl polymer to produce a non-gelled epoxy ester. The epoxy ester is subsequently dispersed into water followed by further coreacting of available epoxy and carboxyl groups on the preformed epoxy ester to form a microgel product. In commonly assigned U.S. Pat. No. 5,508,325, aqueous dispersed microgel polymers are produced by dispersing carboxyl functional acrylic grafted epoxy resin into water followed by dispersion of liquid diepoxide and then crosslinking by the diepoxide. Similar aqueous dispersed diepoxide crosslinked microgel polymeric compositions are disclosed in related U.S. patents U.S. Pat. No. 5,464,885, U.S. Pat. No. 5,554,671, U.S. Pat. No. 5,526,630 and U.S. Pat. No. 5,526,361.
It now has been found that excellent aqueous dispersed protective coating compositions exhibiting improved film integrity properties can be prepared in a similar fashion based on a polymeric binder comprising an aqueous dispersed acrylic-epoxy copolymer, modified with high molecular weight, branched, multifunctional epoxide resin. In accordance with this invention, a branched, high molecular weight epoxy resin is added to the acrylic-epoxy copolymer at the point of dispersing into water rather than pre-mixing the branched epoxy with acrylic-epoxy copolymer. In this regard, the branched epoxy is first mixed with minor amounts of a co-solvent, preferably heated moderately, whereupon the heated mixture is dispersed directly into the previously emulsified acrylic-epoxy copolymer. In has been found by this process that the branched epoxy does not cause an excessive viscosity increase which otherwise occurs due to esterification when mixed directly with the carboxyl functional acrylic-epoxy copolymer prior to forming a dispersion in water. Undesirable esterification reactions ordinarily would cause a considerable increase in viscosity depending on the temperature of the resin mixture and the resident time prior to dispersion into water. However, when the branched epoxy is post added to a previously dispersed (emulsified) carboxyl functional polymer, the viscosity increase can be minimized. After dispersion into water, the branched multifunctional epoxide resin is heat reacted with carboxyl acrylic-epoxy copolymer to form aqueous dispersed crosslinked microgel polymer particles. If desired, low molecular weight diepoxide may also be dispersed into the aqueous polymeric mixture and subsequently coreacted with carboxyl functionality on the dispersed acrylic-epoxy copolymer to crosslink and form aqueous dispersed microgel particles. The microgel emulsion prepared in this way is a stable, aqueous dispersion of small particle size microgel and is useful as a polymeric binder for coatings and particularly exhibiting excellent smoothness, clarity, gloss, and water resistance.
In accordance with this invention, high molecular weight, branched, multifunctional epoxide resins can be dispersed into aqueous emulsions of acrylic grafted linear epoxy resins without undesirably increasing the viscosity of the undispersed resin mixture or increasing VOC's to provide highly resistant sanitary can coatings to a wide variety of foods. The branched epoxy increases the weight average molecular weight of the acrylic-epoxy resin while avoiding the extremely high viscosities which result from adding the branched epoxy prior to the let-down of the resin into water. Upon aqueous dispersion, the branched, high molecular weight epoxy resin is heat reacted and crosslinked with the carboxyl acylic-epoxy copolymer, which not only raises the molecular weight of the acrylic modified linear epoxy copolymer, but effectively reinforces the crosslinked resin mixture with attendant superior resin properties. The advantages achieved with added branched epoxy resin containing higher levels of oxirane functionality include a coating having a stronger polymer network along with improved flexibility, better substrate adhesion, and greater food resistance. Further advantages are realized by reducing or eliminating the need for melamine crosslinkers and the reduction or elimination of phenolics in sanitary food coatings. These and other advantages of this invention will become more apparent by referring to the detailed description of the invention and the illustrative examples.